18) E

The double bond between 2nd and 3rd carbon atom becomes a single bond. The carbon atoms that were part of the double bond will each have a methyl group attached (CH3). This is only shown by option E.

In A, there are no methyl groups.

In B, there are methyl groups on every alternate carbon atom. So this is wrong.

In C, not all carbon atoms part of the polymer chain have a methyl group.

In D, not all carbon atoms part of the polymer chain have a methyl group.